1. Field of the Invention
This invention relates to bicycle frames and their method of manufacture and, more particularly, to a composite bicycle frame formed of composite components that are each separately formed by a method of the invention to have a thin wall with a high strength to weight ratio, and are designed to be easily fitted and secured together into a light, strong and stiff bicycle frame.
2. Background of the Invention
Until recently, bicycle frames have been constructed of metal tubes joined at their ends as by welding, or are brazed or soldered onto metal lugs, forming the frame. Recently, composite materials have been utilized in place of metal for frame construction. Such composite materials have a lower density, higher specific strength and stiffness, and better damping qualities than traditional metals, and thereby provide an increase in frame strength and stiffness with a reduction in weight, as compared to earlier metallic frames.
In construction of bicycle frames using such composite materials, the most common method for joining frame tubes, formed from such composite materials to each other, has consisted of adhesively joining the tubes to metal lugs or joint structures, thereby forming the bicycle frame. The disadvantage of using metallic lug components is their weight relative to composites, the weight of which metallic lug components significantly exceeds the weight of the frame composite tubes, thereby greatly limiting potential weight reductions. Also, the relatively high material density of metallic lug components has tended to favor a use of smaller diameter tubes and hence smaller lugs for weight savings, constituting a lesser strength frame with diminished damping qualities than would be provided utilizing larger tubes. The material density characteristics of metallic lugs has also prevented the development of structurally efficient large gussetted aerodynamic shapes for the lug components on account of the weight increase inherent with such shapes.
Where manufacture of one-piece all-composite unitary bicycle frames has been implemented, such manufacture has been found, for one thing, to be labor intensive. Further, the use of lower than optimum compaction pressures in actual practice has reduced material strength, and has required a greater use of materials than anticipated, resulting in a greater frame weight than predicted. Due to a frequency of structural strength problems and structurally related cosmetic problems occurring in service, such frames have been found to be generally unreliable.
3. Prior Art
Recently, a number of methods and procedures have been developed for producing all-composite bicycle frames. In one such method that provides for secondarily joining precured composite tubes to each other in a desired configuration, the tube ends are joined to each other utilizing uncured composite materials that are wrapped between the tube ends. The joints are then cured in place, forming a connection between the thus joined frame tubes to provide a composite frame. Such method is detailed in U.S. Pat. No. 5,019,312 to Bishop et al.; and U.S. Pat. No. 4,900,048 to Derujinsly. These patents teach similar processes for composite frame fabrication producing frames that are generally not hollow at the tube connections, in which processes the individual tube ends are trimmed or mitered in the joint forming process to fit closely together and receive uncured composite material wrapped therearound. Such uncured composite material as is used in frame construction is generally a reinforcing material that is laid over and between the frame tube members and is cured. To obtain an acceptable appearance for such joint, a significant amount of parasitic material is added to the joint surface for shaping that joint, which parasitic material is essentially an un-reinforced resin, containing fibers and a filler, to control its viscosity, and is for filling gaps and spaces as are present in the joint structure. These large gaps or spaces lie between the external surface on the gusset of the joint and the tubes which lie well inside the joint. Accordingly, the built up joints formed by a practice of such processes increases the weight of the resulting frames.
Additional problems with the above set out type of wrapped joint bicycle frame composite construction is that it is labor intensive at each stage of the manufacturing process, and it is not amenable to mass production techniques and, accordingly, this type of manufacturing process has not been successfully utilized for producing commercially affordable composite bicycle frames.
In a bicycle frame, it is well known to this art that stress loads are always the greatest at joints, and therefore, joint construction has always been a primary determinate influencing frame design and construction. To avoid inherent problems of material discontinuity at frame joints, a number of patents have been directed to reducing or eliminating the number of joints in a bicycle frame through a use of composite materials molded and cured together into a jointless frame. Some examples of this type of composite frame construction are shown in U.S. Pat. No. 3,375,024 to Bowden; U.S. Pat. No. 3,833,2343, to Thompson, Jr.; U.S. Pat. No. 4,015,854 to Ramond; U.S. Pat. No. 4,230,332, to Porsche, U.S. Pat. No. 4,493,749, to Brezina; U.S. Pat. No. 4,856,801, to Hollingsworth; and U.S. Pat. No. 4,986,949, to Brent J. Trimble, which Bowden, Ramond, Porsche, Hollingsworth and Brezina patents all show arrangements for reducing the number of joints in a bicycle frame, with the Hollingsworth patent showing a multi-layer composite unitary frame, and the Thompson, Jr. patent showing a solid cast unitary frame.
Trimble U.S. Pat. Nos. 4,986,949, 4,889,355, 4,902,458, 4,850,607, 4,923,203, 4,941,674 and Reissue U.S. Pat. No. 33,295, and a U.S. patent to Duplessis et al. U.S. Pat. No. 4,828,781, all show examples of jointless or unitary all-composite bicycle frames. These frames are generally hollow, so as to minimize weight as compared to metal or solid or nearly solid composite frames, as set out above.
Unitary composite bicycle frames have somewhat alleviated the problems of extra weight as has been associated with earlier frame joints since the joints are in essence eliminated. The manufacture of a jointless frame in a single step using high performance composite materials to eliminate steps in the manufacturing process, particularly the step where all the components are formed into a single frame. In practice, however, the manufacture of high quality, reliable one-piece, jointless frames has proven difficult and expensive, and accordingly, the number of one-piece jointless composite bicycle frames that have been sold is a small fraction of the number of metal lugged composite tubed frames sold. For example, one large impediment involves the difficulty of reliably producing uniform or high compaction pressures in the composite laminate during cure, due in part to the failure to develop reliable internal pressure bladders to operate satisfactorily throughout the frame. This results in lower structural performance and an outer surface finish which requires a large amount of manual labor to repair. This necessitates, therefore, in order to provide the required strength, that excess materials be used at the frame joints to avoid voids and flaws as could cause joint or frame failure. Such unitary composite frame construction is therefore labor intensive and results in a great deal of waste materials in both the manufacturing process itself and on account of rejections that fail to meet quality standards.
The present invention utilizes all composite hollow tubes and hollow lugs separately formed by the methods of the invention to have uniform thin walls for fitting and adhesive bonding together, whereby a composite bicycle frame is produced that has minimum weight for the required frame strength and stiffness. Problems with earlier frame construction prove that force concentrations at the joints are of major concern in frame design and fabrication. For meeting this problem, the present invention employs hollow frame tubes for fitting over male extensions or plugs extending from lugs their respective plugs of the invention, except that the lugs of the present invention are hollow and the respective mating surfaces of each hollow tube end and each lug male plug are tapered identically. The arrangement is such that the one tapered surface mates with a corresponding surface to produce a joint capable of transmitting high internal forces across the junction.
Insofar as terminology employed herein is concerned, while both the bicycle frame tubes and lugs made in accordance with the present invention are tubular, the frame component referred to hereinafter as a "tube" has reference to the frame elongate components, such as the familiar top tube, down tube, seat tube, and stay tubes that per se are well known to those experienced in the bicycle arts, while the term "lug" has reference to the hereinafter disclosed tubular structure that forms a particular flame joint with the respective "tubes" involved.